Compound semiconductor sensors, like the ones based on cadmium zinc telluride (CdZnTe) material, for example, are being considered in an increasing number of room-temperature gamma-ray detection applications. Though more convenient and practical when used at room-temperature, these sensors suffer a loss of signal to noise compared to conventionally cooled sensors. Several solutions have been proposed and some successfully adopted in order to compensate for the resulting substantial collection deficiency.
The charges, both electrons and holes, generated by the ionizing gamma radiation are subject to substantial trapping effects. Typically, the holes are limited by a much shorter trapping length compared to the electrons; therefore, their contribution to the induction of charge on the electrodes is negligible and subsequently, the induction depends strongly on the electrons. This dependence on the electrons makes the signals induced on the electrodes strongly dependent on the depth of the ionizing interaction, leading to relevant degradation of spectral signal resolution. Therefore, there is a need for an electrode configuration that makes a device that is sensitive to one kind of charge only, but with a resolution that is independent of the depth of the interaction. Several devices have been proposed in the prior art with some success in an attempt to address this need.
Referring to FIG. 1, a co-planar grids sensor (CPG) 10 was introduced in 1994, as described in P. N. Luke, “Single-Polarity Charge Sensing in Ionization Detectors Using Coplanar Electrodes,” Appl. Phys. Lett. 65:22 (Nov. 28, 1994), which is incorporated herein by reference. A CPG 10 includes a cathode 12, and an anode side 14. The anode side 14 includes two co-planar interdigitally connected grids in place of the conventional single electrode. One of the grids is a collecting grid 16 and the other is a non-collecting grid 18. A small bias voltage is applied across the collecting 16 and non-collecting grid 18 so that all the electrons in proximity of the grids are collected substantially only by the collecting grid 16. As in a conventional single planar electrode device, a high voltage supply supplies a high voltage to the cathode electrode 12, thus generating an electric field in the bulk of the compound semiconductor 23.
When charges (electrons and holes) are generated by an ionizing event 20 at a time of occurrence 22, the electrons move in the bulk 23 of the sensor 10 towards the grids 14, and the much slower holes move toward the cathode 12. An identical signal is initially induced, therefore, on each of the two grid electrodes 16 and 18, as well as on the cathode 12. Once the electrons are in a near vicinity of the grids, they are pulled toward and collected by the collecting grid 16, so that after a time of detection 24, the signals differ. A collecting grid signal 26 and a non-collecting grid signal 28 over a collecting time are compared and a difference signal 30 between the collecting grid 26 and non-collecting grid 28 signals is observed. Since the difference induction occurs only when the electrons are in proximity of the grids, the difference signal 30 is, in a first order, independent of the depth of interaction.
The depth of interaction as used herein refers to a distance of an event from the anode side 14. Therefore, a deep-interacting event 32 is characterized as an ionizing event occurring close to the cathode 12, while a non-deep-interacting event 34 is characterized as an ionizing event close to the grids 14.
By making the detector insensitive to the electrons and holes traveling in the bulk, the resolving capability of this type of CPG sensor is greatly enhanced. On the other hand, the conventional CPG sensor is still limited by the trapping of the electrons in the bulk. The longer the electrons travel in the bulk before they are collected by the grid, the higher is the amount of trapping. Consequently, the charge associated with an ionizing event still shows a residual dependence on the depth of interaction that can substantially limit the resolution of the detector. The effect of trapping is shown with solid lines 36 in the signals of FIG. 1, and can be compared to the ideal case of no trapping effects shown with dotted lines 38. As can be seen in FIG. 1, the conventional CPG sensor does not compensate for the residual deficit in pulse height due to the residual trapping effect on the electrons.
In the prior art, two techniques have been proposed to apply to CPG sensor technology to compensate for the electron trapping effect. One method, referred to as the relative gain compensation technique, includes lowering the gain of the non-collecting grid below unity relative to that of the collecting grid. The other method, referred to as a cathode amplitude signal technique includes keeping the gain at unity and weighting each event by measuring its depth of interaction through measurement of the amplitude of the cathode signals.
The relative gain compensation technique essentially re-introduces a small amount of induction from the electrons traveling in the bulk back into the sensor system. The amplitude of the deep-interacting events is increased, while the one of the non-deep-interacting events is reduced, resulting in a first order compensation of the trapping effects. The optimum value of the relative gain typically ranges between 0.6 and 0.9, depending on the quality of the sensor, on the bias voltage and on the temperature. The gain is normally set by modifying the value of a passive element in the differential amplifier, i.e., a resistor.
Though the relative gain compensation technique advantageously does not require complex electronics, the correction is roughly linear, providing only a first order compensation for the trapping effects. As a result, the method is limited to a small amount of achievable compensation and thus the sensor resolution is minimally improved. A second disadvantage is that, if the relative gain needs to be modified, for example, due to replacement of the sensor or a change in voltage and/or temperature, a hardware change is needed, which is impractical especially for commercial applications.
The cathode amplitude signals technique consists of measuring, along with the amplitude of the difference signal, the amplitude of the cathode signal. By calculating the ratio between the difference and the cathode amplitudes, it is possible to extract the depth of the interaction of the event. The difference signal of the event can be corrected (weighted) for the trapping effect according to the value of the associated depth of interaction. The optimum weighting parameters depend on the quality of the sensor, on the bias voltage and on the temperature.
This technique allows a higher order compensation of the trapping effects, thus achieving a better resolution when compared to the previous technique. A second advantage is that the optimization can be performed via software, with straightforward practical consequences. However, more complex electronics are disadvantageously required, including an additional front-end channel for processing the cathode signal, as well as large and bulky high voltage capacitors for coupling the high voltage cathode to the front-end electronics.
There is a need, therefore, for an efficient method and device for measuring the depth of interaction in co-planar grid sensors to increase resolution of detection without the addition of complex and bulky electronics.